This invention relates generally to well logging methods and apparatus for determining the porosity of subsurface earth formations traversed by a borehole and more particularly, to a method and an apparatus for deriving the effective reservoir porosity in a given geologic province.
In attempting to determine the location of oil and gas situated in subsurface earth formations various parameters must be determined, such as porosity, permeability and lithology of the subsurface formations for a qualitative indicator of the presence or absence of hydrocarbons therein. One property of the subsurface formations which is of particular interest is the porosity. Porosity is the fraction, as a percent, of volume occupied by minute channels or open spaces. The total porosity includes all of the interstices or voids, whether interconnected or not. However, the porosity measurement ordinarily used in reservoir studies is the ratio of the interconnected pore space to the total bulk volume of the formation, termed as effective porosity.
There is wide variation among reservoirs in the size of the individual pores and in the arrangement of the pores with respect to one another. The variations are affected by a number of elements which have happended to the formation since it was deposited including compaction and cementation due to the pressure of an increased load acting upon the reservoir sediments. Compaction and cementation are especially significant in a reservoir having sediment containing shales, clays or colloidal materials. Large amounts of absorbed water are squeezed out of these materials by pressure and because clays and colloids are highly plastic, they flow between the grainss to form a cementing or bonding agent and thereby reduce the porosity.
Compaction of reservoir rock is of two kinds, plastic and elastic. Plastic compaction is the squeezing of the soft accessory minerals of the formation matrix, such as clays, weathered products and colloids, into the open pore spaces as the result of pressure increases with the water being driven out. The result is a loss of porosity, a reduction in permeability and an over-all lessening of rock volume.
Most plastic compaction occurs during the digenesis of the formation, when the high water content is being removed. Long continued pressures undoubtedly maintain the process of plastic pore reduction long after digenesis, though at a progressively slower rate. Thus, in sandstones, plastic compaction is evidenced by the squeezed, strained, and deformed soft minerals, by rearrangement of the grains and by closer adjustment of the same grains to the matrix material.
A rock that has undergone elastic compaction can, when the load pressure is reduced, return at least partially to the original volume. Such return is most likely to occur in a firm sandstone. However, in most sandstone, pore space also permanently decreases with an increase in the weight of the overburden, since they commonly contain clay minerals. These clays are squeezed into the pores held open by the touching sand grains and a closer packing results. Thus, a shaly sand may be expected to have suffered more reduction in pore volume for the same pressure than a clean sand.
As the weight of the overburden increases and persists over geological time, the average shale porosity will continue to decrease. Compaction is greater in clays and shales than in sands because of the plastic nature of the clays and because the clays have been swollen by water absorbed into their particles and by water contained in the molecular structure of their crystal plates.
Accordingly, to determine the location and feasibility of recovery of subsurface hydrocarbons, knowledge of the formation porosity is a necessary element. To determine porosity various logging methods have been derived to yield a qualitative indication of porosity, among them are acoustic logging, density logging, and neutron logging. However, each of the logging methods is adversely affected to some degree by borehole conditions. For example, subsurface gas formations will distort the logging signals obtained using density logging. Additionally, washouts and borehole rigosity will give abnormal readings which tend to obscure useful information approximate to the washout.
Compaction trends based on previous logging data have been developed for geologic areas showing the change of total porosity with relation to depth. These trends can be helpful in estimating the effective porosity of a specific reservoir when no porosity log is available or can be used in evaluating the quality of a porosity log when available. However, these depth trends do not adequately take into account compacting and cementing of specific reservoirs caused by the weight of the overburden. Thus, there has been provided no reliable method for estimating the effective porosity of a specific reservoir where no porosity log is available or for evaluating the quality of the porosity log, when available.
Accordingly, the present invention overcomes the deficiencies of the prior art by providing a method and an apparatus for utilizing information derived for correlation of data for a geological formation to provide an effective porosity log which can be recorded or can be used for comparison with a field record made from an actual logging run to determine the areas of inaccuracy or to correct any such inaccuracy.